You will be introduced to creating clones early on in the prologue. You will have to create a clone in order to crawl underneath the gate to progress. Do not activate the gate just yet though as you can spawn clones infinitely here.

To spawn the clone, press + . Now press and hold again and then continuously tap . This way your clone will be instantly destroyed. Repeat the process 15 or 30 times for your achievement. If you are confused about anything, please view my video below.